i have installed text mining add on. now i am truing to read .txt file with the text file widget from the text mining domain, but it shows the data can not be read? what types of files this text file widget can import?

As far as I know, the 'Text file' loads data from specialized xml files with limited use. You can instead format and load data with the regular File widget.For instance load this file with 'Data/File' widget bookexcerpts.tab an use it as the input for other widgets in the Text category.

After some digging through the code I found out that the '.sgm' files mentioned are for loading of Reuters-21578 text collection (it loads all files .sgm in the containing directory).

Another accepted format (the simpest) is a simple .txt file where there are two lines for one document. The first line contains the path to the file with the text and the second line contains the (space separated) document categories.